1. Field of the Invention
This invention relates to an improvement on an optical apparatus, a focus detecting device and an automatic focusing type camera, having focus detecting means which is capable of detecting a state of focus of, for example, each of a plurality of focus detecting areas within an image plane independently of others.
2. Description of Related Art
The focus detecting device of a single-lens reflex camera was arranged in the beginning to permit focus detection only for the central part of a photo-taking image plane. Hence, focus on an object to be photographed must be detected in the central part of the image plane. In a case where a picture must be taken with a focusing object not located in the central part of the image plane, a photo-taking area must be first shifted to have the focusing object in the central part of the image plane and, after that, the photo-taking area must be shifted again to have the object in a desired position within a picture composition on the image plane.
This method, however, has presented various problems. One problem lies in that, it is difficult to seize a good shutter-operating opportunity for a moving object. Another problem lies in that, since the position of the focusing object at the time of actually taking a shot differs from the focus detecting area, the shot might be taken at an inapposite exposure control value.
To solve these problems, some of focus detecting devices are arranged these days to have a plurality of focus detecting areas provided within an image plane, each being arranged to be used independently of another for focus detection. The number of the focus detecting areas is six or thereabout at the most. Focus is adjusted on the basis of information of focus detection from one of these areas that is selected by operating an operation member or the like.
The focus detecting device of the above-stated kind includes a line sensor consisting of a plurality of photo-electric conversion elements for each of the focus detecting areas. The focus is detected by carrying out a computing operation on an image signal which is time-serially outputted from the line sensor as information on the luminance distribution of the object.
The conventional focus detecting device is briefly described below with reference to FIG. 21 which shows a focus detecting optical system of a phase difference type having five focus detecting areas.
The focus detecting device is composed of a field lens AFFL, a perforated field mask AFMSK, a secondary image forming lens AFDL, a stop plate AFDP and an AF area sensor device SNS which consists of a plurality of line sensor pairs.
The perforated field mask AFMSK is provided with slits AFMSK-1 to AFMSK-5 and is disposed in the neighborhood of an anticipated image forming plane of a photo-taking objective lens which is not shown. Each of these slits AFMSK-1 to AFMSK-5 is arranged to determine a distance measuring field. The stop plate AFDP is arranged to restrict a light flux incident on the lens AFDL and is disposed in a position where an image can be formed by the power of the field lens AFFL approximately at the position of the exit pupil of the photo-taking objective lens.
The secondary image forming lens AFDL consists of four pairs of positive lenses. In the lens AFDL, a pair of positive lenses AFDL-1A and AFDL-1B are arranged to cause a part of an object image defined by the mask slit AFMSK-1 to be formed again approximately on a pair of line sensors SNS-2A and SNS-2B. A part of the object image defined by the mask slit AFMSK-2 is formed again by a pair of positive lenses AFDL-1A and AFDL-1B approximately on a pair of line sensors SNS-1A and SNS-1B.
A part of the object image defined by the mask slit AFMSK-3 is split into two orthogonally intersecting light fluxes, which are projected respectively by a pair of positive lenses AFDL-3A and AFDL-3B and another pair of positive lenses AFDL-4A and AFDL-4B on a pair of line sensors SNS-3A and SNS-3B and another pair of line sensors SNS-4A and SNS-4B.
The output of each of the line sensors is read out as an electrical signal indicating a degree of luminance resulting from illumination. Then, a value representing the state of focus of the objective lens on the object detected from the distance measuring field defined by each mask slit is obtained by a computing operation on the electrical signal.
Further, for an image plane FIARA shown in FIG. 22, the distance measuring field which is defined by the five mask slits can be set, for example, at positions AFP1 to AFP5, as shown in FIG. 22.
FIG. 22 shows the relation of the image plane FIARA to the focus detecting areas as seen by looking into the viewfinder of a camera. In FIG. 22, reference symbol FIMSK denotes a field mask which forms a field area of the viewfinder. The image plane FIARA represents a photo-taking area wherein an object to be photographed is observed through a lens mounted on the camera.
The photo-taking area FIARA have five focus detecting areas AFP1 to AFP5. One of the five focus detecting areas AFP1 to AFP5 is selected by the operator of the camera as desired. These focus detecting areas correspond to the line sensor pairs which form the AF (automatic focusing) area sensor device SNS of the image forming optical system shown in FIG. 21, as mentioned above. Reference symbol FDSP denotes an LCD which is arranged to display photo-taking information within the viewfinder.
The focus detecting device having a plurality of focus detecting areas naturally necessitates some means for selecting one of the focus detecting areas. The known focus detecting area selecting means has been arranged to operate in various manners including, for example, the following methods. In an automatic selecting method, an object located nearest is selected according to the focus state of each focus detecting area and the focus is adjusted for the object. In another method, the visual line of the operator looking into the viewfinder of the camera is detected and a focus detecting area located nearest to the visual line is selected. In a manual method, one of the focus detecting areas is selected as desired by manually operating an operation member.
Further, a device for setting photo-taking information by manually operating a rotary operation member of a camera is disclosed in Japanese Laid-Open Patent Application No. SHO 60-103331. According to the disclosure, a photo-taking information setting device for a camera is arranged to have different phases and increase or decrease a setting value by detecting the phases of two signals which are generated in response to a manual operation on a photo-taking information setting member. The device disclosed includes control means which is arranged to let the setting value vary only when the photo-taking information setting member is driven at least 1/2 of the period of the two signals. This control means effectively prevents a display of information from flickering even if the photo-taking information setting member is operated little by little, so that the operability of the camera can be enhanced.
Further, according to a further known method for setting photo-taking information by a dial rotating operation, a dial disposed on a flat surface of a back cover of the camera is rotated to vary, as desired, control information of varied kinds, such as an amount of exposure correction, a shutter time, an aperture value, etc., with the back cover in a closed state. The dial rotating operation is normally performed with a right hand thumb. Therefore, an operation on a shutter release button with an index finger to keep the button in a state of being pushed halfway and only to its first half stroke position can be carried out without difficulty.
To take advantage of this operation method, some of the cameras of the kind having digital signal input means on the side of a back cover are provided these days with a circuit which is arranged to receive a signal from the digital signal input means on the back cover and to process camera information such as an aperture value, a shutter time, etc., and a switching circuit arranged to automatically switch the camera information settable by the digital signal input means from one kind over to another according to whether the camera is in a state of supplying power in response to a release operation or in a standby state. The arrangement permits setting photo-taking information while the camera is in a state of being held with a finger applied to the shutter release button in the process of taking a shot, so that the operability of the camera can be enhanced by diversifying the functions of the digital signal input means on the side of the back cover. The arrangement also effectively prevents erroneous information setting.
Generally, a shutter operating opportunity of a camera passes in a moment. Therefore, a shutter release must be always kept in a ready state while keeping a desired object under continuous surveillance. Therefore, it is important that a focus adjusting (focusing) device is arranged to be capable of quickly carrying out focus adjustment by accurately selecting a focus detecting area even in the event of a quickly moving object.
In this respect, the conventional focus detecting device mentioned above is capable of easily selecting a focus detecting area even if it is arranged to switch the focus detecting area from one area over to another by one round of operation, because the number of the focus detecting areas is only six or thereabout.
However, in the case of a focus detecting device having numerous focus detecting areas consisting of area sensors, it is difficult to quickly switch one focus detecting area to another. Hence, there arises the problem of missing a shutter operating opportunity.
Further, in a case where a focusing object desired is very small, it is difficult to accurately select a focus detecting area where the tiny object is located.